1. Field of the Invention
The subject invention is related to proximity detectors, and more particularly, to an indirect proximity detector that uses surface acoustic wave (SAW) technology to reduce weight and complexity as compared to prior art sensors to meet the rigorous demands of the aviation industry.
2. Description of the Related Art
In aviation, mechanical subsystems are subject to very harsh conditions, so it is imperative that proper operation be verified. If systems do not operate properly, the consequences can be grave. Aircraft are subject to the Federal Aviation Regulations (FAR), which imposes very strict requirements on system performance. In terms of electrical equipment, FAR requires that equipment must be installed so that operation of any one unit or system of units will not adversely affect the simultaneous operation of any other electrical unit or system essential to the safe operation.
Proximity sensors have long been used to indicate the position of an object (e.g., whether a hatch is open or closed). These proximity sensors are typically the current sensing induction type and heavily armored for isolation from electromagnetic interference (EMI). The proximity sensor generates an electromagnetic field to sense metal objects passing within an inch or two. The heavy isolation armor and wire required are highly undesirable in weight sensitive aircraft design. By one estimate for fixed wing commercial aircraft at the time of filing, each extra pound has a recurring cost of over US $500, a non-recurring cost of over US $5000, and a life cycle cost of US $100,000. For rotary wing craft (e.g., helicopters), the costs are typically twice that of fixed wing aircraft. Yet, a 787 airplane has 150 heavy proximity sensors with over 40 lbs of associated wire.
As the industry looks to alternatives, the currently available options have their own disadvantages. One alternative is voltage sensing capacitive type sensors, which may require heavy isolation hardware due to capacitive coupled voltage due to EMI. Magnetic or optical interrupt switches, although much lighter due to less shielding, are very failure prone in the harsh conditions of aviation. Further, magnetic switches attract ferrous metal filings which inhibit the detection of near/far condition, resulting in false condition reporting. Optical switches suffer from foreign object contamination, such as dirt, which again reduces the performance of the device. Still further, each of these sensors still uses wire to power and report state. Wire requires a number of aircraft interactions for installation, which drives up cost in both design and manufacturability. Additionally, wire is heavy and difficult to troubleshoot when operation fails.
In view of the difficulties noted above, the inventor has recognized the need to reexamine how information is gathered and integrated into the control system of an aircraft. The inventor notes that transition to wireless sensor devices can alleviate many of the prior art difficulties despite the strict requirements of FAR and the harshness of the operating environment. For example, one particular wireless technology is known as the Zigbee protocol. However, Zigbee devices require power not only for the necessary Zigbee controller/router devices but for the sensors as well. Although these power requirements can be met wirelessly (e.g., sensor batteries), this approach is poorly suited to application in an aviation platform because of the associated failure rates and maintenance. Radio Frequency Identification (RFID) is another promising identification method. Although RFID has the capability to have a passive tag, the inductive coupling that is typically used requires close proximity and can perform below aviation standards.
SAW devices are another electronic component that the inventor recognizes as promising for addressing the problems of the prior art. SAW devices generate guided acoustic waves along a surface of the device. SAW devices are generally fabricated on single crystal anisotropic substrates that are also piezoelectric. SAW devices typically include one or more pairs of intertwined interdigital electrodes that form transducers (known as an interdigital transducer or IDT) to convert the electrical signals applied to the device into the electromechanical surface acoustic waves generated in the device and vice versa. SAW sensors are among the most sensitive and widely used physical and chemical sensors in gas and liquid environments. SAW devices have also been regularly used for the passive retrieval of information via a wireless retransmission of interrogative radio frequency (RF) signals. The information can be in the form of a modulated code embossed into the geometry of the device for use as an identification device (RFID) tags or information pertaining to a sensor that is attached as a load impedance. Such SAW devices require no battery for certain operations as the devices selectively reflect an interrogating RF pulse back to an interrogating transceiver with the information adapted into the RF signal.
A RFID SAW tag or sensor typically has a suitable antenna attached to the input/output IDT and receives an RF pulse sent from an interrogation unit. The RF pulse is transformed by the input/output IDT into an acoustic wave that propagates along the substrate. The substrate may have other IDTs or other structures to generate a reflection wave that the input/output converts to an electrical signal broadcast by the antenna. The reflective structures encoded on the piezoelectric substrate can return various information such as an identification number, pressure, temperature, strain and the like.
It would be beneficial therefore and it is the intent of the inventor of the subject disclosure, to provide a robust wireless sensor that is passively powered such as a SAW device and, preferably, functions indirectly to overcome the drawbacks of the prior art.